Fume cupboards



&496357 Feb, 24 1970 D. ELLIS ETAL FUME GUPBOARDS Filed Aug. 16, 1958 o u 6 W ///r// /V V United States Patent O U.S. Cl. 98-115 8 Claims ABSTRACT OF THE DISCLOSURE A fume cupboard of the back extraction type comprising means for introducing auxiliary air into the top and bottom of the cupboard, in a direction such as to induce a flow of air and vapours towards a back extraction point constituted by the lower end portion of a baflie plate which is spaced from the floor of the cupboard and curved in the direction of extraction.

This invention relates to the removal of toxic vapours from a confined space such as a fume cupboard.

In a well designed fume cupboard contaminated a r must be replaced by clean air which should have free access to the space occupied by the contaminated air and should flow into that space in a direction away from the operator. Conditions in the fume cupboard should not be subject to any turbulence which might permit the flow of toxic vapours towards the operator and ideally extraction should be eflicient from all levels in the fume cupboard avoiding dead spaces therein and re-circulation of toxic vapours. In order to minimise loss of heat from the building in which the fume cupboard is installed it iS now well recognised that at least part of the air extracted by the cupboard should be supplied from an auxiliary source outside the building.

Existing fume cupboards fail to meet the above criteria to the extent required. Thus in one design auxiliary air is blown out into the general laboratory atmosphere through louvres placed above the operator's head and the proporton in which the auxiliary air is fed to the laboratory and to the fume cupboard itself depends upon the position of the fume cupboard sash which operates a by-pass in a manner varying with its position. In this design all the air entering the fume cupboard below the sash must flow past the operator's body which acts as a deflector and the resulting air flow is unsatisfactory in this respect. In another design the auxiliary air is introduced into the fume cupboard through slots at the top and bottom just behind the vertical line of the sash and a vertical curtain of moving air is maintained across the entrance to the cupboard with the intention of forming a barrier between the operator and the toxic vapours inside the cupboard. Smoke tests with this design show that the air curtain is effective in preventing toxic vapours from the cupboard from entering the laboratory but within the cupboard itself considerable turbulence and re-circulation of toxic vapours is bound to take place thus providing a hazard to an operator who requires to insert part or whole of his person for even a limited time into the fume cupboard space. In both these designs extraction takes place from the back of the cupboard from a space formed between the back wall of the cupboard proper and a vertical plate spaced away from the wall, such fume cupboards being known as the back extraction" type.

We have now improved and modified fume cupboards of the type described above in such a Way as to purge the fume cupboard toxic vapours very effectively by introducing a front to back air flow under conditions in which turbulence is avoided and the efliciency of the new design 3,496,857 Paterted Feb. 24, 1970 of fume cupboard is so high that it may be operated without a sash if desired. According to this invention a fume cupboard of the back extraction type comprises means for introducing auxiliary air into the top and bottom of the cupboard, in a direction such as to induce a flow of air and vapours towards a back extraction point constituted by the lower end portion of a baffie plate which is spaced from the floor of the cupboard and curved in the direction of extraction.

It will be understood that it is not the intention of this inventon to create an air screen but rather to create a smooth purging flow from front-back of the fume cupboard and it has been found that the formation of the lower end of the baffle plate into a curve has a marked effect on the flow pattern in the fume cupboard. Without such curvature it is found that there is considerable turbu- -lence created at the lower edge of the baflle. The introduction of auxiliary air takes place at an angle to the horizontal, as distinct from a vertical air curtain, and for best results it has been found desirable to inset into the floor and roof of the fume cupboard the respective auxiliary air inlet slots. The appropriate direction of the inflowing auxiliary air is Conveniently achieved by providing deflector plates extending over the whole length of the slots and directed towards the back of the cupboard 'at an acute angle to the horizontal. This angle may be varied within wide limits 'but an inclination in the range of 10-30 has been found to give excellent results in practice. When inset slots and deflector plates are used it is found that the injector effect of the incoming air, tendng to set up re-circulation to a limited extent, is mininised. Further, auxiliary air introduced at an angle to the horizontal in this way is not deflected by the body of an operator standing in the normal working position.

The nvention is illustrated in the accompanying drawing which is a vertical section of the fume cupboard in side elevation. The fume cupboard is supported on a platform 1 and is formed With a top section 2 and a base section 3 which are of hollow Construction to provide air boxes 4 and 5 respectively for the passage of air under pressure as hereinafter described. The cupboard has a back 6 and spaced inwardly from it is a baffle plate 7 which has no perforations or louvres as in prior constructions and extends downwardly from the top of the fume cupboard to form an obstruction to gas flow which thereby diverts the gases downwardly and upwardly into an extraction space 8. Connected to the top of the back 6 and the upper extension of the baflle plate 7 is an outlet pipe 9 connected to a fan (not shown). a

The shell of the fume cupboard is completed by side walls (not shown) which are of double wall Construction to provide a connection between the upper air box 4 and the lower air box 5. The plate 7 extends along the width of the fume cupboard but falls short of the side walls on each side leaving a small gap through which gases may pass At its lower end plate 7 is formed with a portion 10 curved through an angle of leading into an upwardly directed flange 11.

Connected to the upper air box 4 is an auxiliary air supply pipe 12 connected to a source of pressurised air (not shown). The air box 4 is formed with a downwardly directed extension 13 at the front of the fume cupboard which terminates in a slot 14 through which auxiliary air is supplied. The inner wall of the extension 13 is formed with an inwardly directed flange 15 which together with a glass plate 16 and a ceiling plate 17 connected at its inner end to the batfle plate 7 form the ceiling proper of the fume cupboard extending across its entire width. The glass plate 16 is readily removable from the position shown to provide access to a light fittng 18.

The outer wall of the air box 4 extends downwardly as shown beyond the slot 14 and the flange 15 and is then turned inwardly to form a deflector plate 19 disposed at an angle of approximately 30 to the horizontal. The slot 14 is inset slightly from the ceiling of the fume cupboard so that pressurisedair passing through the slot and deflectcd by the deflector plate 19 enters the working space in the fume cupboard at somewhat lower velocity in order to avoid undesirable injector effects which could cause some re-circulation near the ceiling of the cupboard.

The floor 20 of the fume cupboard, made in suitably resistant material, slopes rearwardly towards a drain 21 for the remo-val of liquids spilt by accident during operation. Inset into the floor 20 is a slot 22 connected to the lower air box and extending from the outer ledge 23 of the floor 20 is a further deflector plate 24 mounted so as to be directed inwardly at a corresponding angle to the deflector plate 19.

The fume cupboard maybe constructed in a variety of materials and its Simplicity of design lends it readily to production as a unit shell in suitable thermo-plastic materials. For a relatively small laboratory fume cupboard 3 ft. wide (91 cm.) Very good extraction is found to occur when the lower end of the bame plate 7 is about 14 in. to

15 in. (35-38 cm.) above the floor 20 of the cupboard and the curved end is 2 in. (5 cm.) in diameter running into a short upwardly directed flange about 4 in. (10 cm.) long and parallel to the line of the baflie. The gap between the plate 7 and the side walls is usually of the order of 1 in. (2.5 cm.). Preferably the amount of auxiliary air supplied is such that 60% of the total air and vapours extracted by the fan is provided by the auxiliary air.

The direction of air and vapour flow in the normal working of the fume cupboard is shown by the arrows from which it will be seen that no screening effect takes place but a smooth and effective purging from all parts of the fume cupboard independently of the position of the operator.

Fume cupboards according to the invention can be conveniently produced and transported as a kit of parts, suitable for assembly and installation where required.

We claim:

1. In a fume cupboard of the back extraction type having a floor, a celing, a rear wall, side walls, and a front access opening, top inlet means for introducing auxiliary air into the top of the cupboard near the front; a deflector plate extending downwardly and rearwardly toward the back of the cupboard at an acute angle to the horizontal for directing the auxiliary flow of air towards the back of the cupboard as it enters the cupboard through said top inlet means; floor inlet means for introducing auxiliary air into the bottom of the cupboard near the front; a deflector plate extending upwardly and rearwardly toward the back of the cupboard at an acute angle to the horizontal for directing the auxiliary flow of air towards the back of the cupboard as it enters the cupboard through said bottom inlet means; and a baflie plate extending downwardly across the -back of the cupboard forming a back extraction point and having a lower end portion that is spaced from the floor and curved in the direction of extraetion.

2. The fume cupboard of claim 1, in which the top and bottom inlet means comprise elongated inlet slots extending along the front of the ceiling and floor of the cupboard respectively, and the deflector plates extend inwardly and rearwardly from in front of the length of the slots.

3. The fume cupboard of claim 2, in which the inlet slots are inset from the inner surface of the ceiling and floor.

4. A fume cupboard according to claim 1, in which the deflector plates are directed towards the rear of the cupboard at ar angle of from 10-30 to the horizontal.

5. A fume cupboard according to claim 1, in which the lower end portion of the baffle plate is curved through an angle of 180.

6. A fume cupboard according to claim 5, in which the curved portion of the baffie plate leads to an upwardly directed flange.

7. A fume cupboard according to claim 1, in which a gap is provided between the side walls and the baffle plate on each side through which gases may pass.

8. A fume cupboard according to claim 1, in Which the amount of auxiliary air supplied is such that of the total air and vapours extracted from the cupboard is provided by the auxiliary air.

References Cited UNITED STATES PATENTS 2,649,727 8/1953 Snow et al. 98-115 2,702,5(l5 2/1955 Nelson 98--115 3,1ll,077 11/1963 Cortright 98-115 WILLIAM E. WAYNER, Primary Examiner 

